Match List - I with List - II.

Choose the correct answer from the options given below:

(A)-(III), (B)-(IV), (C)-(I), (D)-(II)

The correct answer is Option (2) → (A)-(III), (B)-(IV), (C)-(I), (D)-(II)

A. Morgan introduced the term "linkage" after conducting dihybrid cross experiments involving the fruit fly Drosophila. During these crosses, he noted that two genes did not always assort independently as described by Mendel's laws. His observations revealed that when two genes are located on the same chromosome, the occurrence of the parental combination is notably higher in the subsequent generation compared to the non-parental combination.

B. When a single gene can exhibit multiple phenotypic expression such a gene is called a pleiotropic gene. The underlying mechanism of pleiotropy in most cases is the effect of a gene on metabolic pathways which contribute towards different phenotypes. An example of this is the disease phenylketonuria, which occurs in humans. The disease is caused by mutation in the gene that codes for the enzyme phenyl alanine hydroxylase (single gene mutation). This manifests itself through phenotypic expression characterised by mental retardation and a reduction in hair and skin pigmentation.

C. Down’s Syndrome : The cause of this genetic disorder is the presence of an additional copy of the chromosome number 21 (trisomy of 21). This disorder was first described by Langdon Down (1866). The affected individual is short statured with small round head, furrowed tongue and partially open mouth. Palm is broad with characteristic palm crease. Physical, psychomotor and mental development is retarded.

D. Multiple allelism refers to a genetic condition where more than two contrasting characters are controlled by multiple alleles at a single genetic locus. With multiple alleles, there are more than two potential phenotypes based on the various dominant or recessive alleles present in the trait, as well as the inheritance patterns these alleles follow when combined. An excellent illustration of multiple allelism in humans is the ABO blood type system.